Photographically useful silver halide emulsions, other than high (&gt;90 mole %) iodide emulsions, which are seldom used for photographic purposes, contain silver halide grains that exhibit a face centered cubic crystal lattice structure. Face centered cubic crystal lattice structure silver halides can take different crystallographic forms, depending of the crystal faces by which they are bounded. J. E. Maskasky, "The Seven Different Kinds of Crystal Forms of Photographic Silver Halides", Journal of Imaging Science, Vol. 30, No. 6, Nov./Dec. 1986, pp. 247-254, states that there are seven possible crystal planes or families of crystal planes that can bound face centered cubic crystal lattice structure silver halide grains. These are cubic or {100}; octahedral or {111}; rhombic dodecahedral or {110}; trisoctahedral or {hh1}; tetrahexahedral or {hk0}; icositetrahedral or {h11}; and hexoctahedral or {hk1} grain faces. The descriptive name is derived from the geometrical form of regular grains bounded only by the stated face while the numerical name is the Miller index of the crystal face. h, k and 1 are integers, where h is larger than k and 1 is smaller than h.
Despite the theoretical availability of different crystal forms, photographic silver halide grains rarely exhibit any crystal faces other than {111} or {100} crystal faces. The variety of shapes that silver halide grains exhibit is much more a function of crystal irregularities than variations in grain face crystal planes. Silver chloride grains show a strong propensity toward forming cubic grains bounded entirely by {100} crystal faces, although both cubic and octahedral grains of all face centered cubic crystal lattice structure silver halides are known as well as cubo-octahedral grains. That is, grains with six {100} faces and eight {111} faces, sometimes also referred to as tetradecahedral grains. In addition irregular grains, such as clam form grains (believed to contain a single twin plane) of the type discloses by U.K. Patent Specifications 1,447,307 and 1,529,440; acicular or rod like grains; and multiply twinned grains are all known.
In the 1980's a marked advance took place in silver halide photography based on the discovery that a wide range of photographic advantages, such as improved speed-granularity relationships, increased covering power (both on an absolute basis and as a function of binder hardening), more rapid developability, increased separation of native spectral sensitization imparted imaging speeds, and improved image sharpness in both mono- and multilayer formats, can be realized by producing emulsions in which greater than 50 percent of total grain projected area is accounted for by tabular grains. With the exception of {100} silver bromide tabular grains, tabular grain emulsions contain tabular grains with {111} major faces. They are believed to result from the incorporation of two or more parallel twin planes. Tabular grains are characterized by two parallel major faces that are much larger than any remaining crystal faces of the grains.
Silver bromide tabular grains are the most easily prepared, following by silver iodobromide, silver chlorobromide and silver chloride tabular grains in that order. Although high (&gt;50 mole percent) chloride tabular grain emulsions are known, some difficulties have arisen. Whereas tabular grains have {111} major faces, silver chloride prefers to form grains having {100} faces. Thus, there has been a tendency of high chloride tabular grains to revert to non-tabular forms--i.e., grain stability has been a problem. This problem has been overcome by employing morphological modifiers in preparing high chloride tabular grain emulsions, as taught by Tufano et al U.S. Pat. No. 4,804,621; Takada et al U.S. Pat. No. 4,783,398; and Maskasky U.S. Pat. Nos. 4,400,463 and 4,713,323.
In addition to the specific prior art discussed above it is noted that Lapp et al U.S. Pat. No. 4,379,827; Mifune et al U.S. Pat. No. 4,419,443; Hotta et al U.S. Pat. No. 4,764,457; Ogi et al U.S. Pat. No. 4,868,102; Nishikawa et al U.S. Pat. No. 4,952,491; Japanese Kokai 59-214029; Japanese Kokai 63-2409844; and WO 83/02338 (EPO 96,727 corresponding) were placed of record and considered by the Examiner in the parent application, but not selected as forming a basis for rejection.
Recently increased interest has developed in high chloride emulsions. The much higher solubility of silver chloride as compared to silver bromide offers processing advantages, and there are indications that effluents from processing high chloride emulsions can be reduced and more easily managed to satisfy rising ecological protection standards.
With increased interest in high chloride photographic emulsions an unsatisfied need has been identified. That need is for a high chloride photographic emulsion that has the advantages of tabular grain emulsions, but offers the morphological grain stability of cubic or {100} grain face high chloride emulsions.